Hard-deprecate LegacyHookPreAuthenticationProvider class

Hard-deprecate LegacyHookPreAuthenticationProvider class

The hooks that used to be called by this class will be removed in
I24d6fa963. The only reason to keep this class around is that
someone might have added it to $wgAuthManagerConfig so removing
it would trigger class lookup failures, so make sure any use
of the class triggers a deprecation warning.

Change-Id: I9755288eda7461ecf3dcd35de2081fbb3eb04ae3